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SPOL


Brief Description
SPOL (Available): An imaging spectropolarimeter fed by the MMT's f/9 beam that combines polarizing optics and a transmission-optics spectrograph.  The current gratings provide spectral resolutions of ~4-15 Å over the useful spectral response of 3800-9000A. The instrument is capable of detecting linear or circular polarization at a level p < 0.05%. In the imaging mode, polarization maps can be obtained in filtered bandpasses at the seeing limit over a field-of-view of 19" square. Please contact Paul Smith if you are interested in using this PI instrument.
The Fundamental Capabilities

Quantity

Imaging FOV

19 arcseconds

Resolution range

4 - 15Å

Wavelength range

3800 - 9000Å

Pixel scale

0.19"/pix

Peak Throughput

~36% @ 5500 - 6000Å

Further Information

SPOL is an imaging spectropolarimeter that can be configured with either a λ/2 or λ/4 rotatable achromatic retardation plate to provide linear or circular polarization measurements, respectively. A thin Wollaston prism, which has very high throughput and polarimetric efficiency, is used as the polarization analyzer. In spectroscopic mode a number of slit widths and gratings are available.  For the imaging mode, a large square 19" x 19" aperture is used and a flat mirror replaces the grating. The two polarized beams that emerge from the prism are reimaged as either complementary parallel spectra (spectroscopic mode) or complementary polarized images (imaging mode) on the CCD detector. The polarimetric efficiency (the degree of polarization measured for a completely polarized source) exceed 95% over a range of 4500-8000Å.

The instrument and camera are controlled with a single PC that is programmed to coordinate motion control (shutters, wave plates, etc) and image acquisition. The aperture plate is tilted to provide reimaging to a CCD guide camera for target acquisition and guiding.

An "observation" is the measurement of a Stokes parameter in a sequence of exposures, with polarimetric modulation occurring following readout via rotation of the wave plate. Individual frames are co-added in computer memory according to wave plate orientation, producing two complementary images in orthogonal senses of polarization. These are compared during data reduction to obtain the degree of polarization Q, U, or V.

Data reduction is performed using custom IRAF scripts that are available from the PI. These scripts are comprehensive and straight forward to use and therefore non-polarimetrists can achieve robust results. For information please see the SPOL website.